1. Field of the Invention
This invention relates generally to a filtration material, and more particularly to a filtration material holding an electrostatic charge and comprising three types of fibers blended together.
2. Description of the Related Art
It is known in the filtration art that various kinds of fibers can be formed into a web or other structure having tortuous paths between the fibers through which a gas stream, such as air, is passed to remove particulate matter from the gas stream. The particulate matter in the gas flowing through the paths in the web is retained on the upstream side of the web, or within the tortuous paths of the web due to the size of the particles relative to the paths"" diameters.
Furthermore, it is known to charge various blends of fibers electrostatically to further retain particulate matter through electrostatic attraction between the fibers and the particles. Such blends and other filtration improvements are shown in U.S. Pat. No. 6,328,788 to Auger, U.S. Pat. No. 4,798,850 to Brown, U.S. Pat. No. 5,470,485 to Morweiser, et al., and U.S. Pat. No. 5,792,242 to Haskett, all of which are incorporated herein by reference.
The Auger patent discloses a blend of two types of fibers: polypropylene and polymetaphenylene isophthalamide, such as, for example, is sold under the trademark NOMEX. The preferred ranges of weight ratios polymetaphenylene isophthalamide to polypropylene) is between 5:95 and 50:50 and more preferably between 10:90 and 30:70.
The Brown patent discloses a blend of two types of fibers: polyolefin and a substituted additional polymer. The first fiber in Brown is preferably polypropylene and the second is modacrylic. The preferred range of surface area ratios (polypropylene to modacrylic) in Brown is 30:70 to 80:20, and more preferably 40:60 to 70:30.
The prior art filtration materials provide sufficient filtration for many environments. However, where the electrostatic charge must be retained for long periods or where the penetration through the filtration material must be below a particular percentage, the prior art does not suffice. Therefore, the need exists for a filtration material that provides the needed performance at a feasible cost to manufacture.
It has been discovered that a combination of three fibers can be blended to form a filtration material that has superior performance to the filtration materials of the prior art. The three fiber types include polypropylene, polymetaphenylene isophthalamide, and a third type selected from the group of modacrylic and acrylic. In a preferred embodiment, the amount of polymetaphenylene isophthalamide is between about 5 and 20 percent, and the balance is divided between polypropylene and modacrylic. In another embodiment, the polymetaphenylene isophthalamide content is between about 5 and 20 percent, and the balance is divided between polypropylene and acrylic.